1. Field of the Invention
The present invention relates to a flexible, high-purity expanded graphite sheet, and to a method of producing the same.
2. Description of the Prior Art
Expanded graphite sheets are generally produced by treating natural graphite, pyrolytic graphite, kish graphite or the like with a mixed solution containing sulfuric acid and nitric acid, for instance, then washing the same with water, drying the same, treating the same for expansion in an expansion oven at about 1,000° C., and forming the same into sheets using a rolling machine, for instance. Expanded graphite sheets are excellent in heat resistance and in gas and liquid impermeability and therefore are used as packing materials, valve sheets, gaskets, and fuel cell separators, among others.
Japanese Patent No. 2,620,606 discloses that highly pure expanded graphite sheets having an impurity content of not more than 15 ppm can be obtained by treating such expanded graphite sheets for increased purity in a halogen gas atmosphere at 2,000° C. or above. Such sheets are used in the process of semiconductor production.
Hereinafter, a detailed description is given of a high-purity expanded graphite sheet for use in semiconductor production, which is taken as an example. Such a high-purity expanded graphite sheet is also used in the Czochralski (hereinafter referred to as “CZ” for short) process, which is a representative single crystal pulling technique. A sectional view of the main parts of a CZ apparatus is shown in FIG. 1. The CZ apparatus comprises such parts as a carbon crucible 5 supporting a quartz crucible 1, a heater 2, an upper ring 6, and an inner shield 7, among others. In the CZ apparatus, polycrystalline silicon placed in the quartz crucible 1 is heated to a high temperature to give a silicon melt 3, and the tip of a seed crystal held by a seed chuck is brought into contact with the raw material melt 3 and then pulled up while maintaining the contact to thereby pull up a silicon single crystal 4.
As shown in FIG. 1, the carbon crucible 5 made of graphite or a carbon fiber-reinforced carbon composite material (such crucible is hereinafter referred to as “carbon crucible”) is in direct contact with the quartz crucible 1 and, therefore, the surface of the carbon crucible 5 is gradually converted to silicon carbide thereinafter referred to as “SiC”) as a result of the reaction between the quartz crucible 1 and carbon crucible 5 and/or the reaction between vaporized silicon and the graphite crucible. The difference in coefficient of thermal expansion between carbon and SiC is conducive to cracking of the carbon crucible, for instance. Furthermore, the quart crucible 1 becomes firmly sticking to the carbon crucible 5, making it difficult to take out the quartz crucible 1.
Japanese Patent No. 2,528,285 discloses, as a means for solving such problems, the use of a high-purity expanded graphite sheet as a liner intervening between the quartz crucible 1 and carbon crucible 5.
When an expanded graphite sheet is treated for improving the purity thereof, the flexibility of the expanded graphite sheet is generally impaired, so that it can no loner be used as a member which is required to have flexibility. Therefore, Japanese Patent No. 2,620,606 discloses a method of restoring flexibility which comprises compression molding. However, the method has other problems; the purity of the high-purity expanded graphite sheet is decreased upon compression molding and, when a complicated shape is given to the high-purity expanded graphite sheet insufficient in flexibility restoration by working with a cutter, for instance, the peripheral parts of the sheet are subject to cracking and/or chipping.
When the whole inside surface of the carbon crucible is covered with an expanded graphite sheet, the efficiency of heating of the quartz crucible 1 decreases. Therefore, in recent years, various complicated liner shapes have been proposed so that the quartz crucible heating efficiency may be improved. Basically, each single crystal production operation consumes one intervening liner and, therefore, it is important to provide a method of producing high-purity expanded graphite sheets which is excellent in mass productivity.
Accordingly, it is an object of the present invention to provide a high-purity expanded graphite sheet having flexibility and a method of producing the same. Another object of the invention is to provide a method of manufacturing expanded graphite sheets which is suited for mass production as well.